1. Field of the Invention
This invention relates to a process and apparatus for thermally and pneumatically treating granular solids to remove organic and inorganic materials which are bonded to the granular solids or in admixture with them to provide a purified granular solids product which is suitable for reuse in industrial processes or for other productive uses, such as landfill.
2. Description of the Prior Art
Granular solids are extensively used in industrial processes, particularly in the molding and casting of metals to be used as building materials and machine or equipment parts. Foundry sand comprises granular solids and is commonly used to make casting molds for metals. Various types of binders and hardeners, comprising organic and inorganic materials, are bonded to the foundry sand for use with different types of molds. It is important that the organic or inorganic material coats substantially the entire surface of each granular solid particle so that particles adhere to one another to form a strong mold. The organic bonding agents commonly used with foundry sand are natural drying oils and synthetic resins such as urea formaldehyde, phenolformaldehyde, alkyd isocyanate, phenolic isocyanate, polyester urethane and furans with and without organic acids.
As the production of cast metal products has increased in recent years, consumption of foundry sand or suitable granular solids has increased and has made treatment enabling reuse of the solids important. The accumulation and disposal of waste solids also creates problems. Disposal of large quantities of used foundry sand or other granular solids is particularly difficult when the solids have been treated with harmful or toxic material. These waste solids can no longer simply be dumped or buried in view of stricter environmental standards enforcement. Reclamation of used granular solids provides: conservation of resources, environmental pollution control, and economy of operations.
Methods for reclaiming used granular solids can be generally categorized as wet reclamation, dry reclamation and thermal reclamation. In wet reclamation systems, the used granular solids are washed and agitated in an aqueous solution to rid the particles of chemical additives. Dry reclamation systems utilize scrubbing techniques to remove chemical additives. In thermal reclamation systems, the used granular solids are subjected to high temperatures where the chemical additives are thermally oxidized.
Thermal reclamation processes for removing organic and inorganic additives from granular solids, particularly foundry sand, have received considerable attention. Thermal reclamation processes utilize thermal energy to remove organic chemical additives from the used granular solids by thermal oxidation. Thermal treatment alone will not, generally, remove all inorganic chemical additives, such as bentonite and fine clays which are bonded to or mixed with the granular solids. These inorganic chemical contaminants must be removed by additional processing, usually by scrubbing, after the granular solids have been cooled. Thermal reclamation systems consume large quantities of energy and the reclamation processes have not been economically feasible, particularly in the United States.
Heat exchange techniques to reduce fuel consumption in thermal reclamation processes have been proposed. T. Itoh and N. Suzuki, "Low Energy Thermal Reclamation in Japan", AFS Transactions Vol. 88 (1980) proposes a heat exchanger to recover heat from particles exiting a single stage fluidized bed calciner. Alternatively, they propose a multi-compartment fluidized bed calciner in which sand, after it is calcined, is transferred to a lower chamber where heat is exchanged between the calcined sand and the incoming air. The Itoh et al reference teaches cooling the sand from 800.degree. C. (1472.degree. F.) to 500.degree.-600.degree. C. (932.degree. to 1112.degree. F.) in the lower, heat exchange chamber. The sand cooled in this manner is subsequently further cooled, and scrubbing and screening procedures are performed downstream from the fluidized bed calciner to remove any inorganic chemical additives. According to the Itoh et al reference, used sand is preheated to 700.degree. C. (1292.degree. F.) before it enters the fluidized bed calcining chamber.
Reclamation of used sand, particularly clay bonded sand and furan resin bonded sand in a fluidized bed roaster is reported by T. Watanabe et al, "Reclamation of Used Sand by Fluidized Bed Roaster", Mitsubishi Heavy Industries, Ltd., Technical Review 39 (February 1980). They observe that wet reclamation and dry reclamation methods alone are inadequate for foundry sand because residual organic chemical additives remain bonded to the foundry sand and cause casting defects and rough casting surfaces. Watanabe et al describe a fluidized bed roaster which removes substantially all organic chemical additives. The sand must, however, undergo further processing such as scrubbing, to remove inorganic chemical additives such as clay.
J. J. Geremia, "Thermal Sand Reclamation", AFS Current Information Report (1981) describes the process and suitable apparatus for thermal sand reclamation in a rotary kiln, a multiple hearth furnace, and a fluidized bed system. The fluidized bed calciner according to the Geremia reference comprises a calcining bed and one or more exchange beds for heat recovery. The multi-stage fluidized bed taught by Geremia requires a separate drying compartment for preheating the sand and incorporation of dry scrubbing techniques to remove inorganic chemical additives entailing the further steps of cooling the calcined sand and then subjecting it to pneumatic scrubbing in a separate, downstream pneumatic chamber.
U.S. Pat. No. 4,283,015 discloses an apparatus for removing No-Bake coatings from foundry sand by discharging the sand against a transverse target. Fines are separated and are entrained in an airstream while sand grains of desired particle size pass through the airstream and are recovered.
U.S. Pat. No. 2,478,461 teaches a method of scrubbing solids after they have been discharged from a furnace wherein sand is entrained in high velocity opposing air jets directed against one another so that non-carbonaceous materials are dislodged and removed.
U.S. Pat. No. 2,813,318 claims a method of removing carbonaceous as well as non-carbonaceous coatings from sand by impinging the sand entrained in an airstream against a fixed target.
U.S. Pat. No. 2,707,314 teaches a method for removing non-carbonaceous materials from decarbonized sand which utilizes a centrifugal flinger to impact sand against a rigid target with clean sand falling as a curtain through streams of air to remove foreign material.
U.S. Pat. Nos. 2,456,769, 2,547,587, 3,871,438, and 2,433,738, teach generally the removal of organic materials from foundry sand by burning.